Hydraulic control circuit

ABSTRACT

The present invention relates to a hydraulic circuit including: a variable displacement pump, a booster pump, a pump control controlling the displacement of the variable displacement pump, supplied by the booster pump via pressure control means, the pump control including a double-acting cylinder including a first chamber and a second chamber, wherein the first chamber is connected to said booster pump via a first setting line including pressure reducing means, the second chamber is connected to said booster pump via a second setting line including a proportional pressure reducer, such that control of the proportional pressure reducer controls the displacement of the variable displacement pump.

GENERAL TECHNICAL FIELD

The present invention relates to the field of control means for variabledisplacement hydraulic pumps.

It finds particular application for controlling circuits for driving acooling circuit fan, in the case of cooling a vehicle heat engine forexample.

STATE OF THE ART

Variable displacement hydraulic pumps are widely used in hydraulic powersupply circuits.

These variable displacement hydraulic pumps typically have a variabletilt swashplate, the tilt of this swashplate causing the pumpdisplacement to vary.

The tilt of the swashplate is typically controlled by a single-actingcylinder; this cylinder includes a setting spring and ispressure-controlled directly by tapping the pressure delivered by thepump through a pressure reducer.

Document US 20100132352 shows such a hydraulic circuit, including avariable displacement hydraulic pump, the output whereof is controlledas stated previously.

Such a pump displacement control circuit is an open circuit; some fluid,typically oil, is tapped from the hydraulic circuit and either led tothe cylinder control or exhausted through a leak. Fluid is thus tappedfrom the delivery constantly provided by the hydraulic pump, whichgenerates losses.

In addition, the hydraulic pump is driven by a heat engine, and acontrol circuit involves driving the hydraulic pump in rotation as soonas the engine starts, which causes difficulty when starting it.

Moreover, the control of the displacement of the hydraulic pump in theevent of failure of the control means, for example in the event offailure of the electrical components of the system, poses a safetyproblem.

PRESENTATION OF THE INVENTION

The present invention aims to propose a control circuit for thedisplacement of a hydraulic pump that does not exhibit suchdisadvantages.

To this end, a hydraulic circuit is proposed including:

-   -   a variable displacement pump supplying fluid to said circuit,    -   a booster pump,    -   a pump control controlling the displacement of said variable        displacement pump, supplied by the booster pump via pressure        regulation means,        said circuit being characterized in that said pump control        includes a double-acting cylinder including a first and a second        chamber, wherein    -   the first chamber is connected with said booster pump via a        first setting line having pressure reducing means,    -   the second chamber is connected to said booster pump via a        second setting line having a proportional pressure reducer,        such that the control of the proportional pressure reducer        controls the displacement of the variable displacement pump.

As a variation, said pressure reducing means include two flow reducersassembled in series, defining an intermediate pressure which suppliessaid first chamber.

According to another variation, said proportional pressure reducer isprovided with an electrical load control.

As a variation, a flow reducer is located between the proportionalpressure reducer and the second chamber of the cylinder.

This circuit finds a particular application in a system wherein saidvariable displacement pump supplies a motor for driving a coolingcircuit fan.

As a variation, said variable displacement pump is an axial piston pump,and the variable displacement is controlled by the tilt of a cam plate.

As a variation, said circuit is provided with a calibrated valve tappingthe pressure of said circuit, designed to provide a pressure leak fromsaid circuit when the pressure is greater than or equal to a definedvalue.

PRESENTATION OF FIGURES

Other features, aims and advantages of the invention will appear fromthe description that follows, which is purely illustrative and notlimiting, and which should be read with reference to the appendeddrawings, wherein:

FIG. 1 shows a control circuit of a variable displacement pump accordingto one aspect of the invention.

FIGS. 2 and 3 show variations of the circuit shown in FIG. 1.

FIG. 4 shows an example of a hydraulic circuit implementing the circuitpresented in the foregoing figures.

FIG. 5 illustrates a variation of the circuit shown in FIG. 4.

In all the figures, common elements are designated with the samenumerical labels.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of the hydraulic circuit according tothe invention.

In the hydraulic circuit as shown, a hydraulic pump 1 and a booster pump2 are driven by a common drive shaft 3 which is typically driven inrotation by a heat engine M.

The hydraulic pump 1 supplies hydraulic fluid, oil for example, to ahydraulic circuit C, and has a variable displacement, typically in theform of a variable tilt swashplate 4 the tilt whereof is controlled by apump control 5, typically a cylinder 5.

The hydraulic pump 1 is typically an axial piston pump the variabledisplacement whereof is controlled by the tilting of a cam plate. Thehydraulic pump 1 is designed to circulate the hydraulic fluid in ahydraulic circuit C in two circulation directions, depending on the tiltof the cam plate of the pump 1, the hydraulic pump 1 always being drivenin the same direction. Two ranges of values are thus defined for thetilt of the plate 4 of the pump 1; a first range of values correspondingto a first fluid circulation direction in the hydraulic circuit C, and asecond range of values corresponding to a second fluid circulationdirection in the hydraulic circuit C. Zero displacement is achieved foran tilt of the plate 4 of the pump 1 at the transition between the firstand the second range of values.

The booster pump 2 supplies a boosting circuit G of the hydrauliccircuit C as well as the pump control 5. The booster pump 2 draws thenecessary hydraulic fluid, for example, from a reservoir not shown inthis figure.

In this embodiment, the pump control 5 is a cylinder including a chamber52, a rod 53 which controls the tilt of the plate 4 of the pump 1, andan internal partition 54 connected to an elastic return means 55 such asa spring.

The chamber 52 of the cylinder 5 is supplied by the booster pump 2, viapressure regulation means, typically a proportional pressure reducer 21,provided with a control 22.

The control 22 is typically controlled by means of an electronic controlunit.

The proportional pressure reducer 21 leads the fluid coming from thebooster pump 2 to the second chamber 52 of the cylinder 5 and/or towarda reservoir R which is at substantially ambient or atmospheric pressure,depending on the action exerted by the control 22 on this proportionalpressure reducer 21.

An elastic return means 23 and an equalizing line 24 oppose the actionof the control 22 on the proportional pressure reducer 21, such that,when the control 22 applies a zero force on the proportional pressurereducer 21, the proportional pressure reducer 21 adjusts the pressure ofthe chamber 52 so that it is equal to the pressure in the reservoir R.

In this particular embodiment, the pump 1 is at full displacement bydefault, and the user can use the control 22 in order to lead fluid intothe chamber 52 of the cylinder 5 and thus to alter the displacement ofthe pump 1 by lowering it to 0, and by continuing to increase thepressure in the chamber 52 of the cylinder 5 in this direction up tofull reverse displacement.

In the event of a breakdown of the control 22, the pump 1 is at fulldisplacement, which ensures the operation of the hydraulic circuit Csupplied by the pump 1.

FIG. 2 shows a hydraulic circuit for controlling a variable displacementpump according to a variation of the invention.

In this embodiment, the pump control 5 is a double-acting cylinder,including a first chamber 51 and a second chamber 52, a rod 53 thatcontrols the tilt of the plate 4 of the pump 1, and two internalpartitions 54 connected by an elastic return means 55 such as a spring,said internal walls being located between the two chamber 51 and 52.

The two chambers 51 and 52 of the cylinder 5 are supplied by the boosterpump 2, via pressure and flow control means.

The first chamber 51 of the cylinder 5 is supplied with pressure by thebooster pump 2 via pressure reducing means, including two restrictions11 and 12 assembled in series and producing singular pressure drops. Therestrictions 11 and 12 are typically flow limiters with drainage of theexcess flow, also called sprinklers. As a variation, the restrictions 11and 12 can also be replaced by a pressure reducer as will be shownlater.

The first restriction 11 taps fluid distributed by the booster pump 2,while the second restriction 12 opens into a reservoir R at ambientpressure, typically atmospheric pressure.

The restrictions 11 and 12 assembled in series define an intermediatepressure in the portion of the hydraulic circuit between these tworestrictions 11 and 12, which is led to the first chamber 51 of thecylinder 5. The intermediate pressure defined by the two restrictions 11and 12 has a value comprised between the pressure delivered by thebooster pump 2 and the pressure in the reservoir R.

The second chamber 52 of the cylinder 5 is supplied with pressure by thebooster pump 2 via a proportional pressure reducer 21, provided with acontrol 22, as previously described.

The pressures in the two chambers 51 and 52 of the cylinder 5 opposeeach other, and the position of the cylinder 5 resulting from thesepressures defines the tilt of the plate 4 of the hydraulic pump 1, andhence its displacement. When the pressures within the two chambers 51and 52 are equal, then the cylinder 5 is in an equilibrium position andthe plate 4 of the hydraulic pump 1 defines a zero displacement.

By increasing the pressure in one or the other of chambers 51 and 52 ofthe cylinder 5, the cylinder 5 is displaced which drives the plate 4 ofthe hydraulic pump 1 in a given direction of rotation of the cam plateof the pump 1, which corresponds either to a first delivery direction orto a second delivery direction in the closed loop.

For example, consider an initial condition wherein the chambers 51 and52 of the cylinder 5 are both at a pressure P0. For example uponstarting the system, P0 can be equal to zero. Upon starting the boosterpump 2, the pressure in 51 will increase.

By increasing the pressure within the first chamber 51 of the cylinderof the cylinder 5, for example to a value P1 such that P1>P0, thedisplacement of the hydraulic pump 1 is increased in a first deliverydirection of the closed loop of the hydraulic circuit C corresponding tothe first range of values of tilt of the plate 4 of the hydraulic pump1. Then, by controlling the control 22 of the proportional pressurereducer 21, the pressure within the chamber 52 is made to vary to as toattain a value P2 such that P2>P1. The displacement of the hydraulicpump will then decrease until it has zero displacement when the twochambers 51 and 52 are at pressure P1, then as the pressure continues toincrease in the chamber to reach the value P2, the tilt of the plate 4of the hydraulic pump 1 reaches the second range of values thatcorrespond to the second fluid circulation direction in the hydrauliccircuit C, the delivery whereof increases as the pressure in the chamber52 increases. The hydraulic pump 1 then supplies the hydraulic circuit Cin the second delivery direction, opposite to the first deliverydirection mentioned previously.

The displacement of the pump 1 is therefore controlled thanks to a pumpcontroller 5 constituted here by a double-acting cylinder 5, suppliedwith pressure by the booster pump 2 through pressure control means.These pressure control means therefore enable a user to regulate thedisplacement of the variable displacement pump 1.

When the drive shaft 3 is not driven in rotation by the heat engine M,and the system is therefore stopped, the hydraulic pump 1 and thebooster pump 2 do not supply the circuit C and the booster circuit Gwith fluid.

The pressure in the first chamber 51 and the second chamber 52 is thenequal to the ambient or atmospheric pressure, the control 22 applied tothe proportional pressure reducer 21 then having no effect.

The cylinder 5 is thus in a position of equilibrium, corresponding to atilt of the plate 4 for which the displacement of the hydraulic pump 1is zero.

Upon starting the system, the heat engine M is started in order to drivethe drive shaft 3 in rotation.

The drive shaft 3 drives in rotation the hydraulic pump 1 and thebooster pump 2.

The booster pump 2 then delivers a boost pressure, which will supplywith pressure the booster circuit G as well as the chambers 51 and 52 ofthe cylinder 5.

The chamber 52 is supplied with pressure by the booster pump 2 dependingon the action of the control 22 on the proportional pressure reducer 21.

The chamber 51 is supplied with pressure by the booster pump 2 accordingto the flow rates defined by the flow limiters 11 and 12, and theintermediate pressure that they define.

The establishment of pressure by the booster pump 2 is notinstantaneous; the pressure builds progressively in the lines supplyingthe chambers 51 and 52 of the cylinder 5.

The delivery coming from the booster pump 2 which is established between11 and 12 defines an intermediate pressure. This intermediate pressureis applied to the first chamber 51 of the cylinder 5. A variation inpressure at the inlet of restriction 11 will cause a progressive changein the flow rates through the restrictions 11 and 12 until a new dynamicequilibrium is reached, which will define a new pressure between therestrictions 11 and 12. If the delivery is zero, the pressure is that ofthe reservoir R.

Consequently, upon starting the hydraulic pump 1, the two chambers 51and 52 of the cylinder 5 are at ambient pressure for a given duration,during which the cylinder 5 is in a position of equilibrium and thedisplacement of the hydraulic pump 1 is nil. The torque to be suppliedwhen starting the heat engine M to drive the hydraulic pump 1 istherefore nil or substantially nil.

Once the booster pump 2 has established pressure in the chambers 51 and52 of the cylinder 5, the tilt of the plate 4 of the hydraulic pump 1 isaltered, which causes the hydraulic pump 1 to begin delivery.

By way of example, considering that the heat engine M starts withinsubstantially 2 seconds of a start instruction from a user, the timeneeded for the booster pump 2 to build up boost pressure can be chosento be substantially equal to 2 seconds, the hydraulic pump thereforeremains at zero displacement for substantially 4 seconds from the startcommand. In this manner, starting of the heat engine M is facilitated.This time can be adjusted by selecting the sizing of the restrictions 11and 12.

FIG. 3 illustrates a variation of the circuit illustrated in FIG. 2,also including a restriction 14 (commonly called a “sprinkler”) locatedbetween the proportional pressure reducer 21 and the second chamber 52of the cylinder 5.

In this variation, the restriction 14 makes it possible to damp theentering and leaving flow in the second chamber 52 of the cylinder 5 inorder to avoid a too sudden resumption of delivery of the hydraulic pump1.

Circuits such as those described previously find a particularapplication for controlling fans performing the cooling of a radiator ofa heat engine.

By way of an example, vehicles such as trucks, buses or urbantransportation vehicles, public works machinery, agricultural machineryor lifting machines, which include a heat engine and a radiatorrequiring cooling, can be mentioned.

FIG. 4 illustrates an example of the application of the circuitpresented in FIGS. 1 and 2 applied to the control of such a cooling fan.

In this embodiment, the hydraulic pump 1 supplies the hydraulic circuitC including an hydraulic motor 6 which drives in rotation a shaft 61whereon is assembled a fan 62, typically designed to cool an elementsuch as the radiator of a heat engine.

The tilt of the cam plate of the hydraulic pump 1 determines thedirection of circulation of the fluid in the hydraulic circuit C, andtherefore the direction of rotation of the fan 62.

Thus a high-pressure branch HP of the hydraulic circuit C is defined,which is the branch of the circuit C upstream of the hydraulic motor 6,and a low-pressure branch BP of the hydraulic circuit C which is thebranch of the circuit C downstream of the hydraulic motor 6.

As shown, the booster circuit G includes non-return means 71 providingfor the boosting of the hydraulic circuit C in the event of insufficientpressure, and pressure limiters providing for the drainage of fluid inthe event of overpressure in the hydraulic circuit C. The boostercircuit G also includes a pressure limiter 73 allowing fluid to drain toa reservoir R at ambient or atmospheric pressure when the boost pressureexceeds the desired value.

In the event of a failure of the control 22 of the proportional pressurereducer 21 controlling the pressure in the second chamber 52 of thecylinder 5, the proportional pressure reducer 21 is returned to itsinitial position by the elastic return means 23 and the equalizing line24, such that the pressure within the second chamber 52 of the cylinder5 decreases. Only the first chamber 51 of the cylinder 5 is thensupplied with pressure by the booster pump 2, which results in anincrease of the displacement of the hydraulic pump 1, hence the drivingof the hydraulic motor 6 and the fan 62.

The control circuit as proposed therefore makes it possible to ensurethe operation of the fan 62 in the event of a failure of the control 22,the hydraulic pump then be advantageously at maximum displacement todrive the hydraulic motor 6 which drives in rotation the fan 62.

FIG. 5 illustrates a variation of the circuit presented previously inFIG. 4.

This figure illustrates in particular the reservoir R from which thebooster pump 2 draws hydraulic fluid in order to inject it into thehydraulic circuit C.

By way of example, the circulation direction of the hydraulic fluid inthe hydraulic circuit C has been shown with arrows at the inlet andoutlet of the hydraulic pump 1 and the hydraulic motor 6. Ahigh-pressure branch HP of the hydraulic circuit C and a low-pressurebranch BP of the hydraulic circuit C are thus defined.

The high-pressure branch HP corresponds to the branch of the hydrauliccircuit C upstream of the hydraulic motor 6, while the low-pressurebranch BP corresponds to the branch of the hydraulic circuit Cdownstream of the hydraulic motor 6.

It is easy to understand that in the event of a change in the directionof circulation of the fluid in the hydraulic circuit due to the controlof the plate of the hydraulic pump 1, the branches HP and BP arereversed.

In the circuit illustrated in FIG. 5, the first chamber 51 of thecylinder 5 is connected to the booster pump via a pressure reducer 13defining a constant pressure. According to a particular embodiment, saidpressure reducer 13 can be associated with one or more restrictions aspreviously presented in FIG. 2.

The second chamber 52 of the cylinder 5 is, for its part, connected tothe booster pump 2 through a proportional pressure reducer 21 equippedwith a control 22 and allowing a variable pressure to be applied in saidsecond chamber 52 of the cylinder 5. In the embodiment shown, thisproportional pressure reducer 21 is assembled in series with arestriction or sprinkler 14.

The pressure in the chambers 51 and 52 of the cylinder 5 is determinedaccording to the control and/or setting of the pressure reducers 13 and21.

In the embodiment shown, the second chamber 52 of the cylinder 5 is alsoconnected to the hydraulic circuit C, for example at the outlet of thehydraulic pump 1 via a pressure limiter 25 provided with setting meansthat can be controlled.

This pressure limiter 25 enables the implementation of protection of thehydraulic circuit C, and more precisely a protection of the hydraulicpump 1. Indeed, in the case where the pressure in the high pressurebranch HP of the hydraulic circuit C exceeds the set value of thepressure limiter 25, this pressure limiter 25 directs some flow into thebranch between the restriction of the sprinkler 14 and the secondchamber 52. The flow creates a pressure drop when passing through thesprinkler 14, thus creating a pressure in the second chamber 52, whichdecreases the tilt of the plate 4.

The tilt of the plate 4 of the pump 1, and hence the displacement of thehydraulic pump 1, is thus decreased, which makes it possible to avoidoverloading the hydraulic pump 1 or more generally the hydraulic circuitC. Unlike prior solutions which consist of creating a leak or losses inthe hydraulic circuit in order to limit the pressure there, the proposedcircuit thus makes it possible to obtain maximum efficiency from thehydraulic pump 1.

In this particular embodiment, it is therefore the combination ofcontrol of the proportional pressure reducer 23 and of the pressurelimiter 25 which determines the pressure within the second chamber 52 ofthe cylinder 5.

As shown in FIG. 5, the pressure is tapped at the high-pressure branchHP of the hydraulic circuit C. It will be easily understood that thepressure can also be tapped at the low-pressure branch BP of thehydraulic circuit C, or that a selective tapping can be carried out inone or the other of these branches by means of a shuttle valve.

The embodiment shown in FIG. 5 also includes an exchange line which tapshydraulic fluid from the low pressure branch BP by means of apressure-compensated flow limiter 8 which makes it possible to obtain asubstantially constant flow regardless of the pressure at its terminals,and sends it to a reservoir R.

The booster pump 2 provides for the replacement of this fluid which istapped from the hydraulic circuit.

It is thus possible to tap hydraulic fluid having a high temperature dueto its circulation in the hydraulic circuit C and to re-inject hydraulicfluid at ambient temperature, which makes it possible to avoid thedanger of overheating the circuit.

It will be well understood that in the case where the direction ofcirculation of the hydraulic fluid in the circuit C is reversed, theexchange takes place in a similar fashion because thepressure-compensated flow limiter is able to exchange on an HP branch inthe same manner as on the low pressure branch BP.

The compensated flow limiter could just as well be placed on ether ofthe two branches, high pressure HP or low pressure BP; for betterefficiency, however, it is advantageous to place it on the branch thatwill be the low pressure BP branch in normal operation.

This exchange system is simplified compared with a usual, morecomplicated and costly, exchange valve, connected both to the highpressure HP and low pressure BP lines and selecting the low pressure BPline (for example by means of a slide valve selector) to carry out theexchange.

The invention as previously described thus exhibits several advantages.

Starting of the heat engine M is carried out with the hydraulic pumphaving zero delivery, which therefore greatly facilitates its startingby reducing the torque that it must supply. The start of delivery of thepump 1 occurs a few seconds after starting of the heat engine, forexample in order to provide for its cooling in the case where the pump 1enables the driving of a cooling circuit fan. In this particularapplication, it is well understood that driving the fan as soon as theengine starts is not imperative, due to the fact that the temperaturerise in the circuit to be cooled occurs progressively.

The displacement control circuit of the hydraulic pump 1 also enablesthe implementation of a safety function in the event of failure of thecontrol 22, by providing maximum cooling due to the resulting maximumdisplacement of the hydraulic pump 1.

Control of the displacement of the hydraulic pump 1 does not cause anyloss of power, because instead of controlling a leak at the power outputof the pump in an open loop, a minimum tapping is carried out on theboosting delivery, and not on the drive pump.

The hydraulic and mechanical efficiency is therefore superior.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. A hydraulic circuit including: a variable displacement pump,supplying fluid to said circuit, a booster pump, a pump controlcontrolling the displacement of said variable displacement pump,supplied by the booster pump via pressure control means, said circuitbeing characterized in that said pump control includes a double-actingcylinder comprising a first chamber and a second chamber, wherein thefirst chamber is connected to said booster pump via a first setting lineincluding pressure reducing means, the second chamber is connected tosaid booster pump via a second setting line including a proportionalpressure reducer, such that the control of the proportional pressurereducer controls the displacement of the variable displacement pump. 2.The hydraulic circuit according to the foregoing claim, wherein saidpressure reducing means include two restrictions assembled in series,defining an intermediate pressure which supplies said first chamber. 3.The hydraulic circuit according to claim 1, wherein said proportionalpressure reducer includes an electrical setting control.
 4. Thehydraulic circuit according to claim 1, wherein a restriction is placedbetween the proportional pressure reducer and the second chamber of thecylinder.
 5. The hydraulic circuit according to claim 4, wherein thesecond chamber of the cylinder is connected to the hydraulic pump via apressure limiter, such that the pressure within said second chamberresults from control of the proportional pressure reducer and from thepressure drop created by the flow passing through the pressure limiterthrough the pressure drop.
 6. The hydraulic circuit according to claim5, wherein said pressure limiter is designed to tap fluid in ahigh-pressure branch of said circuit and to lead it into the secondchamber of the cylinder when the pressure in said high-pressure branchof the circuit is greater than or equal to a defined value.
 7. Thehydraulic circuit according to claim 1, wherein the variabledisplacement pump supplies a motor for driving a fan of a coolingcircuit.
 8. The hydraulic circuit according to claim 1, wherein thevariable displacement pump is an axial piston pump, and the variabledisplacement is controlled by the tilt of a rotating cam plate.
 9. Ahydraulic circuit according to claim 1, wherein the circuit is equippedwith a flow reducer designed to provide a hydraulic fluid leak bytapping hydraulic fluid in a branch of said circuit, the booster pumpbeing configured to inject hydraulic fluid into the circuit in order tocompensate for said leak.